Various injection molded products are produced using a process commonly referred to as a “full shot” injection molding process, in which a molten thermoplastic resin is injected into an injection mold cavity in an amount sufficient to fill the cavity (i.e., a “full shot” of resin). However, many specific applications require these products to be produced at a reduced weight while maintaining their respective physical properties.
One way to reduce the weight of an injection molded product is to form the product from a foamable resin. This reduces the amount of resin required to form the product, thereby reducing the overall weight of the product. Foamed injection molded products may be produced using a “short shot” injection molding process, in which a foamable resin is injected into the injection mold cavity in an amount less than the volume of the cavity (i.e., a “short shot” of resin), and the foamable resin is allowed to expand in the cavity. But the products formed by this process lack the aesthetic characteristics of their full shot counterpart, as the foaming resin lacks sufficient force to pack itself against the surface of the cavity.
Gas assist has generally been used to provide pressurized gas for assisting in packing resin against the cavity walls of an injection mold. But gas assist is generally ineffective for use with a foamable resin, as the porosity of the foaming resin allows the gas to pass through the resin like a sponge. Prior attempts to use gas assist with a foamable resin have therefore been limited by the amount of foaming that can occur prior to the use of gas assist. As such, the amount of resin used in the injection molding process must be sufficient (e.g., to prevent the occurrence of a blow out when applying the gas assist), thereby limiting the weight reduction achieved.